Re: Regression in 4.11 - PowerPC crashes on boot, bisected to commit 5657933dbb6e

From: Larry Finger
Date: Thu Mar 09 2017 - 17:22:10 EST

This is a multi-part message in MIME format. On 03/06/2017 03:48 PM, Benjamin Herrenschmidt wrote:
On Mon, 2017-03-06 at 13:46 -0600, Larry Finger wrote:
I was able to create a PPC emulation with debian-8.7.1-powerpc-CD-1.iso
following the instructions in My only
problem was that "-net tap" fails and I did not find any way to get networking

After looking at the screen through a number of crashes, I have determined that
the top entry in the traceback comes from dmam_alloc_coherent(). I have not been
able to see the offset to determine which BUG_ON call in that routine is being

I tried to modify panic() to see if I could keep the screen on longer after the
failure, but no joy so far.

I think the problem is this code in drivers/macintosh/macio_asic.c:

/* Set the DMA ops to the ones from the PCI device, this could be
* fishy if we didn't know that on PowerMac it's always direct ops
* or iommu ops that will work fine
* To get all the fields, copy all archdata
dev-> = chip->lbus.pdev->dev.archdata;
#endif /* CONFIG_PCI */

This is definitely bad. A quick fix is to copy the new dev->dma_ops field
(as well, there is still stuff in archdata that we need too).

A better long term fix is to have a set of macio_dma_ops wrappers that do
"the right thing".


Attached is a patch that fixes the crash. At the moment, it has my s-o-b, but I do not feel it right to claim authorship. My role should be a Reported-and-tested-by. Please advise.